PROJECT SUMMARY/ABSTRACT Acute respiratory distress syndrome (ARDS) is an inflammatory lung disease associated with high morbidity/mortality and limited treatment options. A breakdown of pulmonary endothelial barrier function, leading to edema and impaired lung function, is a hallmark of ARDS. It is well established that stimulation of cAMP synthesis, such as with ?-adrenoceptor agonists, enhances endothelial barrier function and is protective in preclinical models of sterile lung injury such as upon LPS administration. However, clinical trials probing the utility of ?-agonists in ARDS have failed and the reasons remain unclear. Cyclic nucleotide phosphodiesterases (PDEs), the enzymes that degrade and inactivate cAMP, play a critical role in the regulation of cellular cAMP levels, the subcellular compartmentalization of these signals, and hence endothelial cell functions. Our preliminary data indicate that a single Type-4 PDE isoform, PDE4D, contributes the predominant portion of cAMP-hydrolytic capacity in the pulmonary endothelium and is tightly regulated under physiologic conditions. P.aeruginosa (PA) is a common cause of nosocomial pneumonia that can progress to sepsis and ARDS. We observed that during the host-pathogen interaction, distinct PA virulence factors induce PDE4D activation, resulting in a suppression and dysregulation of endothelial cAMP signals. Specifically, the bacterial exotoxin cyclase ExoY promotes a PKA-mediated phosphorylation and activation of PDE4D that alters endothelial cAMP signaling. In addition, PA infection can induce a PKA-independent, but type-3 secretion system- and flagellin-dependent PDE4D regulation that results in activation and subcellular relocalization of the enzyme. With this proposal, we will define the pathways by which PA virulence factors alter PDE4D functions and its contribution to endothelial barrier disruption and lung injury. We will test the idea that PA-induced PDE4D activation correlates with health outcomes in patients with PA-associated ARDS, and that, conversely, inhibition of PDE4 is endothelial barrier protective. We will test the idea that aberrant PDE4 activation limits the therapeutic efficacy of ?-agonists in settings of PA-lung infection as well as other causes of ARDS. The PDE4 family comprises four genes and non-selective PDE4 inhibitors have established therapeutic effects in preclinical models of ARDS, but also induce side effects, such as emesis and nausea, that limit their clinical utility. Given the unique and non-overlapping physiological and pathophysiological roles of each PDE4 isoform, targeting individual PDE4 proteins can serve to dissect the therapeutically beneficial from the side effects of the PAN-PDE4 inhibitors available to date. To this end, we will determine whether selective ablation of PDE4D is protective in ex vivo and in vivo models of PA-lung injury, paving the way for development of PDE4D-selective inhibitors as ARDS therapeutics with an improved safety profile compared to the non- selective PDE4 inhibitors available to date.